In recent years, there is an increasing demand for the advent of an oscillator with high precision frequency stability (e.g. within ±0.28 ppm in the Stratum 3) in a base station for a mobile phone or in a transmission system which needs to meet the Stratum 3 specifications.
A temperature compensated crystal oscillator is utilized in these applications as a reference clock source, and has a trait of reducing frequency changes of a crystal resonator to temperature. This is achieved by controlling temperature characteristics (e.g. its temperature characteristics are approximated using a cubic function in a crystal resonator made of crystal unit quarried out at a cutting angle, which is called AT cut) of a crystal resonator (piezoelectric resonator).
FIG. 15 is a view showing a configuration of a general crystal oscillator.
In FIG. 15, the crystal oscillator includes a crystal resonator SS and an oscillation circuit CC to oscillate the crystal resonator SS. The oscillation circuit CC includes an amplifier A and a resistor R connected in parallel to the crystal resonator SS, a load capacitance element Ca (capacitance value Cca) connected between an input side of the amplifier A and ground, and a load capacitance element Cb (capacitance value Ccb) connected between an output side of the amplifier A and the ground.
The above resistor R is also referred to as feedback resistor, which functions as defining a DC operating point of input and output.
In the aforesaid configuration, if the load capacitance elements Ca and Cb each have a variable capacitance, it will be able to control an oscillation frequency.
FIG. 16 is a view showing an equivalent circuit of the crystal oscillator shown in FIG. 15.
In FIG. 16, a crystal resonator side SSS has a configuration in which a motional capacitance C1 (capacitance value CC1), a resonance resistance R1 (resistance value RR1), a motional inductance L1 (reactance value LL1), and a shunt capacitance CO (capacitance value Cc0).
In the meantime, the oscillation circuit side CCS has a configuration in which a resistance component Rn (resistance value RRn) and a capacitance component CL (capacitance value CCL) are connected in series. The resistance component Rn is a negative resistance component having a negative value. A well-known LC oscillator may be configured by cancelling a resistance value of the resistance component R1 with a resistance value of the negative resistance component.
Specifically, the capacitance component CL is an oscillator equivalent capacitance component of an equivalent circuit. A relationship among the capacitance value CCL of the oscillator equivalent capacitance component CL, the capacitance value CCa of the load capacitance element Ca, and the capacitance value CCb of the load capacitance element Cb is expressed by an equation (1) below:CCL=(CCa×CCb)/(CCa+CCb)  (1)
The equation (1) indicates that when the capacitance value CCa of the load capacitance element Ca is small and the capacitance value CCb of the load capacitance element Cb is small, the capacitance value CCL of the oscillator equivalent capacitance component CL becomes small.
Here, a relationship between the capacitance value CCL of the oscillator equivalent capacitance component CL and an oscillation frequency f is expressed by an equation (2) below:F=½π{LL1×CC1×(CC0+CCL)/(CC0+CC1+CCL)}1/2  (2)
The oscillation frequency f describes a curve as shown in FIG. 17 to the value CCL of the oscillator equivalent capacitance component CL.
Referring to FIG. 17, it shows that the oscillation frequency f drops in substantially inverse proportion to an increase in the capacitance value CCL of the oscillator equivalent capacitance component CL.
Herein, prior art of a voltage controlled oscillator to supply a voltage as a control signal is shown in FIG. 18.
In FIG. 18, it is feasible to raise (or drop) a voltage of a control signal CS to increase (or decrease) the capacitance value CCL of the oscillator equivalent capacitance component, thereby dropping (or raising) the oscillation frequency f (e.g. see Patent Document 1).
In addition, in FIG. 18, if a control signal is input as the control signal CS so as to correct temperature characteristics of an oscillation frequency of the crystal resonator, it will become possible to configure a temperature compensated oscillator.
The temperature characteristics of the oscillation frequency of the crystal resonator made of AT cut crystal are approximated using a cubic function for temperature.
The capacitance value CcL of the oscillator equivalent capacitance component in the above-mentioned equation (2) is so controlled by the control signal CS of the temperature compensated oscillator as to correct the temperature characteristics of the oscillation frequency of the crystal resonator. This enables reducing a change in the oscillation frequency f to temperature.
Here, where an oscillator with high precision below 0.5 ppm is required, in the conventional technique the temperature characteristics of the oscillation frequency of the crystal resonator have been corrected up to a high-order component higher than a third-order component. This enables improving the accuracy more than a case where approximation is made using a cubic function (see e.g. Patent Document 2).
In FIG. 19, here is an example of the temperature characteristics of the oscillation frequency of the crystal resonator made of AT cut crystal unit and an example of the temperature characteristics of the oscillation frequency after temperature compensation in a case where the temperature characteristics are the temperature compensated by the temperature compensated oscillator.
In FIG. 19, concerning the oscillation frequency of the crystal resonator made of the AT cut crystal unit, an oscillation frequency fa at temperature Ta is higher than an oscillation frequency f0 at temperature T0 by Δfa. Because of this, the temperature compensated oscillator shown in FIG. 17 makes its oscillation frequency closer to the oscillation frequency f0 by increasing the capacitance value CCL of the oscillator equivalent capacitance component by ΔCLa from CL0 to CLa, and by dropping the oscillation frequency fa at temperature Ta by Δfa.
Meanwhile, the oscillation frequency fb at temperature Tb is lower than the oscillation frequency f0 at temperature T0 by Δfb. Thus, the temperature compensated oscillator shown in FIG. 17 makes its oscillation frequency closer to the oscillation frequency f0 by decreasing the capacitance value CCL of the oscillator equivalent capacitance component by ΔCLb from CCL0to CCLb, and by raising the oscillation frequency fb at temperature Tb by Δfb.
In the temperature compensated oscillator, the capacitance value CCL of the oscillator equivalent capacitance component, different from temperature to temperature, is controlled by the control signal to make a change to the temperature in the oscillation frequency f small.